Those in the petroleum industry are particularly concerned with extracting petroleum by boring holes into deep underground rock formations. To improve the flow of hydrocarbon fluids into the borehole from the surrounding rock, explosive devices are placed in the borehole and detonated, causing piercement and fracturing of the rock. These explosive devices are called Perforating Guns and contain a series of shaped charges, each with a primer connected by explosive cord called detonating cord. The detonating cord is also connected within the Perforating Gun to a detonator. The explosion is initiated by the detonator and travels along the detonating cord and past the series of shaped charges, detonating each of them in turn, to the last shaped charge in the Perforating Gun. In a successful detonation, all of the shaped charges explode. Occasionally, the explosion sequence will terminate before all of the charges have detonated, against the desire and intent of the operator. In a worst case, none of the explosive charges will detonate even though the operator has activated the firing sequence.
In the current state of the industry's technology, no method is available to the operator that can quickly and reliably provide a quantitative estimate of the extent of the detonation of the Perforating Gun. A quantitative estimate of extent of detonation would be one that provides the operator with the length of the Perforating Gun that detonated or a percentage of the total length of the Perforating Gun that detonated. At best, the operator may get an indication of a probable firing of the gun from a transducer positioned on the well structure at or near the well head. Presumably, the absence of a signal indicates a total misfire. This method often fails to give correct indications of gun firing or misfiring, as the case may be. Moreover, this wellhead transducer method provides neither an indication of a partial misfire nor a quantitative estimate of the extent of detonation.
Tubing-conveyed Perforating Guns (TCP Guns) are typically detonated below the packer and are intended to be permanently left in place as petroleum production ensues after detonation. In the case of TCP the operator thus may never learn from retrieval and direct inspection of the gun that a partial detonation has occurred. He may suffer direct economic loss in that the productive rock formation is only partially perforated and petroleum production from the perforated borehole is correspondingly reduced. A potentially far greater economic loss may stem from the operators resultant under-estimation of the production potential of the oil field, based on the lower production rate after a partial perforation that he believes to be a complete perforation of the formation. Under-estimation of the field's potential could lead to a wrong decision such as to limit further drilling activity or even to abandon the field and the initial investment in that field.
Other types of Perforating Guns may be retrieved to the surface for inspection after attempted detonation. The operator will be able to observe the extent of detonation upon inspection. Even in this method of operation it would be useful for the operator to know, without waiting for withdrawal or inspection, the extent of detonation. If the operator knew immediately of a failed detonation he could undertake appropriate remedial action as best available. Backup detonation means could be activated if available. Knowledge of the existence of unexploded charges could allow the operator to implement procedures designed to enhance safety of workers in this situation.
Direct sensing of the detonation at the location of the gun itself is impractical in the case of TCP operations, in that no wire or cable can be conveniently connected to the Perforating Gun.
Thus, there is a need in the petroleum industry for a method of indirect remote sensing of the detonation with a rapid and reliable determination of the extent of detonation of a Perforating Gun.